In view of providing desirable properties such as corrosion resistance to a member having metal surface such as iron, zinc and aluminum, chemical conversion treatment is being widely practiced for a member having metal surface. The mainstream as a chemical conversion coating obtained by such chemical conversion treatment has conventionally been a so-called chromate coating that contains hexavalent chromium. However, the use thereof is substantially prohibited with consideration for environmental conservation, so the currently used chemical conversion coating is that which does not substantially contain hexavalent chromium but contains trivalent chromium, i.e. a hexavalent chromium-free chemical conversion coating (referred hereinafter simply to as “chemical conversion coating”).
In addition, properties needed for surface treatment are being notably upgraded in recent years. In this regard, finishing treatment may further be performed in many cases for a member having been formed thereon with the above chemical conversion coating in order to provide a corrosion resistance comparable with or higher than that of a chromate coating.
This finishing treatment is an applying-type process and performed in a manner as below. A finishing agent is caused to contact with a member of which the surface has been treated with chemical conversion treatment (referred also to as “member under treatment” in the present invention) to form a liquid film of the finishing agent on the surface, and the member having been formed thereon with this liquid film is dried to form a finishing coating on the member under treatment.
In general, the finishing coating is configured of an organic-based material that forms a matrix of the coating and an inorganic-based material that provides desirable properties such as corrosion resistance. As the inorganic-based material, for example, phosphoric acid or other phosphoric acid-based compound and trivalent chromium are used (e.g. Patent Document 1).
If, however, the inorganic-based material comprises phosphoric acid-based compound and trivalent chromium, then the trivalent chromium elutes out from the formed finishing coating due to the moisture adhering to the finishing coating thereby resulting in a problem such that the finishing coating does not have desired sufficient corrosion resistance. Hereinafter, an ability to suppress such elution of trivalent chromium from the finishing coating is referred to as elution resistance.
Accordingly, proposed conventional techniques include a means for adding zinc to the finishing treatment agent as disclosed in Patent Document 2.